Chronic potassium-deficiency in the rat causes a marked reduction in plasma flow into the inner medulla and papilla (PPF) of the rat kidney. The reduction in PPF may reflect a decrease in blood flow and, also, of glomerular filtration rate (GFR) in juxtamedullary nephrons. Thus, delivery of solute to the ascending limb of juxtamedullary nephrons may be diminished. This sequence of events may, in part, explain the reduction in papillary solute content seen in potassium deficiency, and the contribution which the latter plays to the concentrating defect seen in the potassium-deficient state. The present experiments will study the effect of chronic potassium deficiency in the rat on the intrarenal distribution of blood flow and GFR. In addition, alterations in the renin-angiotensin system and in renal prostaglandins may contribute to the renal hemodynamic abnormalities in potassium deficiency. Therefore, the effect of the acute administration of an angiotensin II antagonist and of an inhibitor of thromboxane synthetase on blood flow and GFR distribution, PPF, and urinary concentration will be studied. Finally, the release of radiolabelled arachidonic acid from tissue phospholipid and the production of renal prostaglandins will be examined in normal and potassium-deficient kidneys. The observed association of renal vasodilatation and natriuresis has suggested a necessary relationship between these two events. Recently, marked renal vasodilation has been observed without a natriuresis. Thus, some unique intrarenal event(s) must occur when a natriuresis is seen in the vasodilated kidney. The present studies, utilizing micropuncture techniques, will characterize differences in segmental sodium transport in the rat kidney as natriuretic (bradykinin, acetyl-choline, prostaglandins) and non-natriuretic (imidazole, secretin) agents are administered. As a decrease in the sodium gradient between juxtamedullary thin ascending limb and the medullary interstitium may be an important determinant of the degree of natriuresis, the effect of the dilating agents on PPF and the sodium gradient mentioned will be examined.